I don't know if I'm the right person to reply but I'll try my best.
I have to do big parties with work often. I don't mean the normal office do. I'm an illustrator and we all tend to work alone, as do authors. All through the year we are invited by our publishers to mix and meet up. I'm a fairly friendly and confident person but even I get very nervous at these things. Speaking over the years to people (even the people hosting) most people seem to feel nervous.
My strategies are, to have at least one or two people I know to have a 'base'. Either I enter with them or arrange to see them there. I have stock sentences/questions rehearsed and try and keep it all cheery. I like to calm myself often in the toilets and then go back out. Unless I'm having an amazing time I go home at a respectable hour always having seen and thanked the people hosting.
I'm normally expected to attend these things, but when I haven't I replied with things like -
Thank you for your lovely invite but I'll be visiting elsewhere at that time.
Thank you for your lovely invite but I have a previous family event.
Etc etc. Little white lies can be good. Never respond with "I really can't myself to go" (although I did invite someone to a book launch who responded with a 'I'm so sorry, I have crippling social anxiety" and I felt only sympathy for her and thanked her for her support anyway.
Most people feel nervous at big parties but hide it well. If you really can't face it, Just go with whatever little white lie you can think of and relax.